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Thermoelectric Cooling

CCD cameras for astrophotography are designed to dramatically reduce the effects of dark current in the CCD chip by cooling the chip. Because CCD cameras usually need to be portable, commercially-sold cameras are not cooled with conventional refrigeration systems.  Rather, they use a thermoelectric cooling module to drop the temperature of the CCD chip. 

Hereís a small dose of physics (sorry, but I actually am a physicist, so I canít resist...): Thermoelectric cooling modules use the Peltier Effect.  The way this works is this: certain semiconductor modules have the ability to achieve a temperature difference between one end and the other, when a DC electric current is applied to the module.  (Typically the semiconductor is made of bismuth and tellurium - a bismuth telluride module.) So you can attach a thermoelectric cooling module to the back of a CCD chip and have a CCD camera with low-enough dark current to enable you to take the longer exposures required to shoot a deep-sky object. And the whole thing ends up very small and portable as long as you have a DC power supply, which can be a small battery system or can be fed by an AC adapter if you are close to your house. (Since you also need a laptop or desktop computer to acquire the images from the camera you need to work out some kind of a power supply anyway.)  Note also that this Peltier Effect semiconductor is also used in the small coolers sold for vehicles, that plug into a lighter socket and claim to keep beverages cold.

Another reason this whole subject is interesting in astronomy is that you can also have the opposite effect, called the Seebeck effect. Here, a particular kind of semiconductor device that is hot at one end and cool at the other, can generate an electric current! This is important if you plan to send a satellite away from the sun, to the outer planets. Going in this direction means that you canít use photovoltaic cells - thereís not enough solar energy at those distances to generate useful power. So when NASA sends a probe to the outer planets it uses a thermionic generator. The heat source itself is actually a radioactive isotope that decays fast enough to generate a lot of heat, in a module surrounded by thermionic cells that use the Seebeck effect to generate electricity from the decay heat.  These thermionic generators are built for NASA by the U.S. Department of Energy.

Does this have any practical application in your life? Well, do you live in a cold climate and own a wood stove? You can purchase a really neat fan that sits on your wood stove, moves the hot air above the stove into the room, and doesnít require a power source! It uses a thermionic module to generate electricity because the top of your stove is hot.  Itís totally passive and works well, and as a matter of fact it doubles as a good indicator of how hot the stove is (by the speed of the fan) allowing you to regulate your stove well.  You can get one at Real Goods, and I heartily recommend this product.

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